Emissions from fossil fuel combustion facilities, such as flue gases of coal-fired utilities and municipal solid waste incinerators, include mercury. The emissions include vaporized mercury as elemental mercury, Hg0, or as part of a mercury-containing compound (e.g., oxidized mercury). The oxidized mercury typically occurs as a form of mercury (Hg+2), such as mercuric chloride or mercuric nitrate.
Many countries either regulate or are contemplating regulations of emissions of mercury within waste gases because of potential environmental hazards posed by the mercury emissions. Hence facilities that generate gas emissions, which may contain mercury, typically would monitor total mercury concentration in the emissions to comply with the regulations. To detect the total amount of mercury present within emissions generated by a facility, oxidized mercury in a gas sample is often converted into elemental mercury, followed by measurement of the total amount of elemental mercury within the gas sample. Several different techniques are used to perform the conversion.
For example, one technique involves the use of a wet chemical solution containing SnCl2 (i.e., a wet chemistry method) to reduce the oxidized mercury of a gas sample into elemental mercury. The technique bubbles a gas emission sample through the wet chemical solution to convert Hg+2 to Hg0. The resulting elemental concentration is the sum of both the oxidized and elemental forms of mercury.
Another conversion technique involves heating an emission sample to temperatures of approximately 750° C. Heating of the Hg+2 within the sample separates or “cracks” the oxidized mercury into an elemental component, Hg0, and an oxidizing component. In certain situations, after cracking the Hg+2 within an emission sample into Hg0 using the relatively high temperature, the facility introduces H2 to react with O2 present within the emission sample. The combination of the H2 with the O2 forms water vapor that, upon immediate collection via a condensing unit, removes the separated oxidizing components or compounds such as HCl and reaction byproducts before they have the opportunity to reoxidize the elemental Hg.